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Description of Business and Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]      
USF contributions and other regulatory costs, before reversal   $ 7,700  
Reversal of accrued federal fees $ 0 (3,114) $ 0
USF contributions and other regulatory costs $ 8,600 $ 4,600 $ 6,200